The invention relates to a voltage controlled oscillator (VCO) useful in phase lock loop (PLL) systems wherein an oscillator is locked in phase with an incoming signal. The commercially available MM74HC4046 (formerly CD4046) is a complementary metal oxide silicon (CMOS) integrated circuit intended for PLL applications up to a maximum operating frequency of about 20 MHz. It consists essentially of a VCO and a phase comparator combination. The phase comparator responds to the VCO output and an incoming signal to produce a d-c output that, after filtering, controls the VCO to produce a phase lock.
In the VCO the timing capacitor is charged through a control circuit in first one direction then the other direction with the charging current being determined by the input voltage. This current is produced by a current mirror in which a fixed component can be determined by a resistor. A variable component related to the input voltage is combined in the mirror to provide the control function.